ldns-testns: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/bin/ldns-testns - from package: ldns

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/ldns-testns - from package: ldnsutils

You may install the required package with command apt-get install {package}

Error in other languages:
ldns-testns: bevel nie gevind nie
ldns-testns: comandă negăsită
ldns-testns: comando não encontrado
ldns-testns: commande introuvable
ldns-testns: command not found
ldns-testns: käsku ei ole
ldns-testns: không tìm thấy lệnh
ldns-testns: komanda nerasta
ldns-testns: Kommando nicht gefunden.
ldns-testns: kommandot finns inte
ldns-testns: komut yok
ldns-testns: nie znaleziono polecenia
ldns-testns: níor aimsíodh an t-ordú
ldns-testns: no se encontró la orden
ldns-testns: no s'ha trobat l'ordre
ldns-testns: opdracht niet gevonden
ldns-testns: parancs nem található
ldns-testns: perintah tidak ditemukan
ldns-testns: príkaz nenájdený
ldns-testns: příkaz nenalezen
ldns-testns: команда не найдена
ldns-testns: командата не е открита
ldns-testns: 命令找不到
ldns-testns: コマンドが見つかりません